[Haskell-cafe] is closing a class this easy?

[Miguel Mitrofanov]

Oops... Sorry, wrong line. Should be

isAB :: forall p. p A -> p B -> p x


On 18 Jul 2009, at 10:51, Miguel Mitrofanov wrote:

> What is it for? Yes, you would know that only A and B are Public,  
> but you have no way of telling that to the compiler.
>
> I usually prefer something like that:
>
> class Public x where
> blah :: ...
> isAB :: forall y. (A -> y) -> (B -> y) -> x -> y
>
> Both solutions, however, allow the user to declare some new  
> instances when GeneralizedNewtypeDeriving is enabled.
>
> On 17 Jul 2009, at 19:38, Conor McBride wrote:
>
>> Friends
>>
>> Is closing a class this easy?
>>
>> --------------------------------------
>>
>> module Moo
>> (  Public(..)
>> )  where
>>
>> class Private x => Public x where
>> blah :: ...
>>
>> class Private x where
>>
>> instance Private A where
>> instance Public A where
>> blah = ...
>>
>> instance Private B where
>> instance Public B where
>> blah = ...
>>
>> --------------------------------------
>>
>> Modules importing Moo get Public and its instances,
>> but cannot add new ones: any such instances must be
>> accompanied by Private instances, and Private is
>> out of scope.
>>
>> Does this work? If not, why not? If so, is this well
>> known?
>>
>> It seems to be just what I need for a job I have in
>> mind. I want a class with nothing but hypothetical
>> instances. It seems like I could write
>>
>> --------------------------------------
>>
>> module Noo
>> (  Public(..)
>> ,  public
>> )  where
>>
>> class Private x => Public x where
>> blah :: ...
>> blah = ...
>>
>> class Private x where
>>
>> public :: (forall x. Public x => x -> y) -> y
>> public f = f Pike
>>
>> data Pike = Pike
>> instance Private Pike
>> instance Public Pike
>>
>> --------------------------------------
>>
>> But if I don't tell 'em Pike, I've ensured that
>> blah can only be used in the argument to public.
>>
>> Or is there a hole?
>>
>> Cures youriously
>>
>> Conor
>>
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>
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